This invention relates to electronic thermometers and more particularly to an integral self-contained thermometer capable of measuring the body temperature of an individual.
The prior art is replete with a number of patents relating to electronic temperature indicators and more particularly to conversion of a temperature. In regard to such prior art techniques, there is a persistent problem in selecting and implementing an optimum sensor device which will accurately respond to the temperature of the human body with a minimum accuracy of .+-.0.2 of 1.degree. F. The prior art utilized many devices which are temperature sensitive, but which have substantial problems in implementing the above described accuracy.
Thermistors which are temperature sensitive devices are extremely nonlinear and in order to operate the same in conjunction with a thermometer, one requires extensive compensating circuitry and elaborate calibration techniques. Devices such as thermocouples have extremely small outputs and create problems in compensating for the cold junction. There are, of course, integrated circuit devices such as temperature transducers which may incorporate diodes or temperature sensitive resistors fabricated from semiconductor materials. These devices, apart from being expensive, require multipoint calibration and are not particularly suitable for measuring temperature within the above described accuracy.
It is, of course, understood that utilizing electronic techniques to measure temperature is, in fact, well known in the art and as indicated, many references disclose pertinent techniques. In any event, there is a need for an electronic thermometer which essentially would replace the conventional mercury thermometer which is now employed relatively extensively throughout the world. In order to provide such a device, one must employ an extremely accurate and reliable sensor which is capable of operating over long periods of time, such as for many years, without recalibration while being extremely rugged and convenient and simple to use. In accordance with such a desire, the electronic thermometer should be relatively small, reliable, while possessing an accurate digital readout of body temperature. The use of the thermometer should be identical to the use of a conventional thermometer as providing temperature measurement by inserting the same into the appropriate cavity of a user. In order to accommodate the long use and rugged requirements, one needs to provide an instrument which will dissipate small amounts of energy to enable the same to operate on a self-contained battery for an extensive period of time. Hence, the circuitry which converts the output of the sensor into a final digital display must be capable of being integrated, must be rugged and accurate in format and structure to accommodate all the above described requirements.
As indicated, the prior art has attempted to specify devices which are electronic and operate to measure temperature. U.S. Pat. No. 3,834,238 entitled ELECTRONIC THERMOMETER issued on Sept. 10, 1974 to F. K. Mueller et al discloses an electronic thermometer having a temperature sensitive probe. The probe is essentially a thermistor which operates in conjunction with suitable compensating circuitry. In the patent, the thermistor operates in conjunction with two optional electronic circuits; each of which include means for changing a basic comparison voltage in increments through a series of reference voltage levels. The output of the circuit monitors the voltage output from the thermistor until the thermistor voltage and a comparison voltage are equal corresponding to the measured temperature.
U.S. Pat. No. 3,906,796 entitled ELECTRONIC TEMPERATURE MEASURING APPARATUS issued on Sept. 23, 1975 to R. F. Dumbeck depicts apparatus where temperature is measured by sensing the voltage across two series resistors. One resistor is a reference resistor and the other a temperature variable resistor fabricated from platinum. The resistors are associated with two matched current sources; each of which passes current in opposite directions towards the junction of the resistors to provide a null at a predetermined temperature. The patent describes linearization techniques which serve to match the temperature characteristic of the variable sensor with a linear voltage scale as, for example, the scale of a digital voltmeter. In this manner, one can read temperature directly via the voltmeter in either degrees centrigrade or Fahrenheit. Tracking is also accomplished over a wide temperature range by increasing the current output from the current sources by a feedback technique.
U.S. Pat. No. 3,946,613 entitled ELECTRONIC THERMOMETER AND PROBE issued on Mar. 30, 1976 to S. Silver. This Patent employs a thermistor probe which is coupled to a bridge circuit and permanently calibrated to the bridge circuit. A similar technique is depicted in U.S. Pat. No. 3,949,609 issued on Apr. 13, 1976. Other Patents which attempt to measure temperature utilize algorithms in order to predict a final temperature output. Such a technique is described in U.S. Pat. No. 3,978,325 issued on Aug. 31, 1976 to H. Goldstein, et. al.
As indicated, many electronic thermometers have been designed for hospital use and are relatively bulky and cumbersome devices. These units cannot be conveniently utilized for home use as they are extremely complicated and expensive. Example of such devices are depicted in U.S. Pat. No. 4,007,832 and U.S. Pat. No. 4,008,614.
With the advent of digital circuitry and in particular the integrated circuit, there has been an attempt in the prior art to utilize digital techniques including a digital display of temperature to indicate the final reading. While many of such techniques are interesting they suffer in that they cannot accurately respond to temperature variation within the tolerances required for typical use. Indicative of such prior art is U.S. Pat. No. 4,009,615 entitled SELF-CONTAINED ELECTRONIC DIGITAL THERMOMETER issued on Mar. 1, 1977 to G. A. Ruhl, U.S. Pat. No. 4,030,363 entitled ELECTRONIC THERMOMETER issued on June 21, 1977 to M. E. Halleck, U.S. Pat. No. 4,068,526 entitled ELECTRONIC THERMOMETER issued on Jan. 17, 1978 to H. Goldstein.
Essentially, devices which employ digital techniques utilize an analog to digital convertor to convert the output of the sensor to a digital signal which is finally displayed. It is of course understood that the techniques in regard to such units vary. For examples of such techniques reference is made to U.S. Pat. No. 4,092,863 entitled ELECTRONIC THERMOMETER CIRCUIT issued on June 6, 1978 to R. Turner, U.S. Pat. No. 4,121,462 entitled TEMPERATURE INDICATING APPARATUS issued on Oct. 24, 1978 to R. Mohraman, U.S. Pat. No. 4,158,965 entitled ELECTRONIC THERMOMETER WITH HEAT CONDITION PROBE issued on June 26, 1979 to H. Prosky, and U.S. Pat. No. 4,161,880 entitled LINEARIZED DIGITAL THERMOMETER issued on July 24, 1979 to H. Prosky.
It is absolutely obvious that there exists a great number of Patents which attempt to monitor temperature and to provide the user with either a digital or analog display. These thermometers employ electronic techniques and hence may be generically referred to as electronic thermometers. In any event the problems of the prior art in providing such a thermometer are significant and as one can ascertain such devices have not been successful due to many factors. As alluded to above, a major problem is a formation of a sensor which is capable of accurately responding to temperature variations with a satisfactory time constant. Another significant problem relates to calibration of the device. For example, if such devices are to be operated by means of a battery or other source of power, one must be sure that voltage variations do not affect the calibration. As one understands, electronic circuitry is sensitive to changes in voltage, temperature, humidity and other environmental factors and many of the prior art devices do not maintain accurate temperature measurement under such varying factors. A further factor to be considered is the type of sensor employed, as most prior art devices employ and use thermistor devices which are nonlinear and have extreme problems in proper calibration.
As indicated above, other typical types of sensor devices which may include thermocouples, integrated circuit transducers as well as, semi-conductor diodes suffer in regard to signal output, calibration and accuracy.
In conjunction with sensor problems, the circuit techniques employed in many of the above indicated patents have problems in regard to the accuracy of readout, power consumption, reliability, which make such techniques particularly difficult to implement for use in a conventional thermometer.
It is therefore an object of the present invention to provide an improved thermometer incorporating a unique sensor configuration to enable the reading of temperature in an extremely accurate manner. A further object is to provide an electronic thermometer assembly which is extremely compact as employing an electronic thermometer assembly which is extremely rugged and which assembly utilizes small amounts of power to enable extended operation in a typical home or hospital environment.